This invention relates to measuring temperature by the use of thermocouples. More particularly, this invention relates to thermocouple measuring devices and methods used to measure the rate of change of the temperature of the thermocouple.
Thermocouples are constructed of two dissimilar metals (typically wires) joined together at a measurement junction. As is well known, thermocouples produce a small output voltage (referred to as the “thermoelectric voltage”) that is proportional to the temperature difference between the measurement junction and a reference junction. The reference junction is often referred to as the “cold junction”. In modern electronic thermocouple measurement devices (thermocouple signal conditioners), the cold junction is usually formed at the location where the thermocouple wires are attached to the copper conductors of a circuit board, connector, or cable.
As is well known, a practical thermocouple measurement device commonly includes a differential input amplifier to increase the small thermoelectric voltage to usable levels, and a means of developing a signal (often referred to as the “cold compensation” signal) which varies automatically with the cold junction temperature. The amplified thermoelectric voltage and the cold compensation signal are summed to obtain a signal proportional to the temperature of the thermocouple measurement junction. These requirements have been met in a variety of ways in prior art. For example, U.S. Pat. No. 4,475,103 describes an integrated-circuit thermocouple signal conditioner that includes amplification, cold compensation, and summing functions on a single chip. U.S. Pat. No. 6,942,382 describes a miniature connector with on-board electronics providing all of the functions needed for a thermocouple signal conditioner.
As previously noted, thermocouple signal conditioners provide a signal that is proportional to the temperature of the thermocouple junction. Usually the object of the measurement is to determine the temperature of a media such as a gas, liquid, or solid that the thermocouple is in contact with. Thus, in order for the measurement to be accurate, steps must be taken to ensure that the thermocouple junction assumes the same temperature as the media of interest. This can be difficult or impossible if the temperature of the media changes rapidly.
For example, when the junction of a thermocouple is immersed in a flowing gas, and the gas undergoes rapid temperature fluctuations, the thermal inertia of the thermocouple may prevent it from changing temperature as rapidly as the gas. This well-known phenomenon is often described using terms such as thermocouple “time lag”, “response time”, and “time constant”. In many cases involving rapid temperature changes, the thermocouple cannot respond fast enough to track the actual temperature of the media unless the measuring junction is extremely small, and therefore fragile and impractical for sustained use. Thermocouples used in industrial, aircraft, and automotive applications are normally encased in a metal protections tube (referred to as a “sheathed thermocouple”) which further slows the response to temperature changes.
In some applications it is desirable to be able to measure not only temperature of the thermocouple junction, but also to measure the rate of change (derivative) of this temperature. This is achieved through the mathematical operation of differentiation, and can be accomplished using well known analog circuits known as differentiators. The derivative of the thermocouple signal can provide information about temperature fluctuations in the media that are too rapid to be tracked by the thermocouple junction temperature.
Furthermore, the temperature signal, the derivative signal, and information about the time constant of the thermocouple can be used to synthesize a signal that approximates the actual temperature of the media during rapid temperature changes. This technique (often referred to as “thermocouple time lag compensation”) is well known in prior art, and was originally developed in the early 1950's by the U.S. National Advisory Committee for Aeronautics. It is disclosed in the following publication:
Shepard, C. E. and Warshawsky, L., “Electrical Techniques for Compensation of Thermal Time Lag of Thermocouples and Resistance Thermometer Elements”, Technical Note 2703, NACA, Washington D.C., May 6, 1952.
Despite being known for over fifty years, the technique of thermocouple time lag compensation has not been widely used. One reason for this is the difficulty in obtaining a high fidelity derivative signal from a thermocouple. Thermocouples with adequate durability for industrial, aerospace, and automotive applications (typically relatively large, sheathed-type thermocouples) may only produce a raw (un-amplified) signal fluctuation of a few microvolts in response to high frequency temperature fluctuations. Thus, the signal is highly vulnerable to contamination by electrical noise.
By nature, differentiation amplifies any high frequency noise that is present. This noise will obscure the portion of the signal that reflects the actual derivative of the thermocouple temperature. Low-pass filtering may be used to reduce this problem, but filtering that is sufficient to remove the unwanted noise may also remove much of the intended derivative signal as well.
The cable or wiring that connects the thermocouple to the electronic circuitry of the signal conditioner is an important source of noise problems. Significant extension cable lengths are required when thermocouples are used to measure high temperatures, because the signal conditioner must be located far enough from the heat source to avoid damage to the electronic components. In order to measure the thermocouple junction temperature accurately, the two wires of the extension cable must be made of materials similar to the two alloys of the thermocouple being used. For example, if the well-known K-type thermocouple is used, the chromel lead of the thermocouple must be extended with chromel cable wire, and the alumel lead of the thermocouple must be extended with an alumel cable wire. This ensures that a cold junction is formed only at the intended location near the cold compensation circuitry.
Thermocouple cable is more prone to noise pick-up from external sources than copper cable, in part because the two conductors have unequal resistance values. The thermocouple cable will also cause noise if the conductors are subjected to strain from flexing or vibrations. This problem (often referred to as a “micro-phonic effect”) is far worse with thermocouple cable than with copper conductor cable.
Thus, there is a need for an improved method and apparatus that will enable high fidelity derivative measurements to be obtained from thermocouple signals.